The present invention relates to drive systems for electric machines and, more particularly, to a four-wheel drive assist incorporating a hydraulic drive for the steering end wheels that is automatically engaged when slipping is sensed or that is selectively engaged by an operator. The system has been developed to produce an electric machine that operates efficiently in an indoor environment while having the ability to operate in an outdoor construction environment, where more terrainability is required.
It is known that the most efficient drive on a battery-powered machine is through the use of an electric motor and gear reduction. This construction is easily installed on the non-steering end of a machine. However, because of the physical length of such a drive, it is impractical on the steering end of a unit where it has to swing through an arc.
Conventional four-wheel drive boom lifts are internal combustion engine powered. The engine drives a pump, which in turn drives four hydraulic drives. The drives are hydraulically connected through flow dividers, which distribute power appropriately to the four wheels. This type of machine achieves its maximum tractive effort by summing the power of all four wheels, providing power to negotiate grades and rough terrain. Much of the time, however, these units operate on smooth or at least improved surfaces, where the performance of full-time four-wheel drive is not required. On the smooth or improved surfaces, the valving and extra motors cause losses in efficiency. The devices which in rough terrain are used to distribute power further reduce efficiency when steering and driving on improved surfaces.
A conventional hydraulic drive fits well and easily on the steering end. A conventional drive, however, reduces efficiency in conditions where four-wheel drive is not required, due to the pumping losses through the motor and hosing.